The present invention relates to medical sensors, and in particular to the signals generated for transmission by such sensors.
Non-invasive photoelectric pulse oximetry is an example of a medical sensor which is well known and is described, for instance, in U.S. Pat. No. 4,911,167, incorporated herein by reference. Pulse oximeters typically measure blood flow characteristics including, but not limited to, blood oxygen saturation of hemoglobin in arterial blood. Pulse oximeters pulse light through body tissue where blood perfuses the tissue and photoelectrically sense the absorption of light in the tissue. The amount of light absorbed is used to calculate the amount of the blood constituent being measured.
FIGS. 1A and 1B together are a block diagram of an oximeter 100 such as the pulse oximeter model N-200 which is commercially available from Nellcor Incorporated, Hayward, Calif., U.S.A. FIG. 1A shows the sensor, patient module and analog front end of the oximeter. A patient sensor 110, for sensing and transmitting the pulsed light, includes a photodetector 112 and a pair of light emitting diodes 114, 116 ("LED's"). Typically, a first LED 114 emits light having a mean wavelength of about 660 nanometers in the red light range and the second LED 116 emits light having a mean wavelength of about 905 nanometers in the infrared range.
The photodetector 112 detects the red and infrared incident light, producing a current which changes value in response to the changes in the intensity of red and infrared light transmitted in sequence. The photodetector current produced has a small magnitude, typically in the range of 1.times.10.sup.-9 amps. Because the current generated by the photodetector is so small, the signal is susceptible to inaccuracies caused by noise. In addition, the low current value generated decreases the degree of precision to which the detected signal can be accurately measured. By amplifying the photodetector current, noise susceptibility is decreased and the degree of precision to which the signal may be accurately measured is improved.
The detected current is converted to a voltage signal 122 and amplified by a combined current-to-voltage converter and amplifier 118 in a patient module 124, which may be separate from sensor 110. The sensor signal on line 122 from amplifier 118 is provided to an analog front-end circuit 120 which receives the amplified analog optical signal on line 122 from the patient module 124 and filters and processes it. The front-end circuit 120 separates the detected signal into red and infrared analog voltage signals 126, 128 corresponding to the detected red and infrared optical pulses. The voltage signal on line 122 is first passed through low pass filter 130 to remove unwanted high frequency components and AC coupled through capacitor 132 to remove the DC component and unwanted low frequency components. The signal is then passed through a buffer amplifier 134 to remove any unwanted low frequencies and a programmable gain stage 136 to amplify and optimize the signal level presented to the synchronous detector 138.
Synchronous detector 138 produces a synchronously-rectified voltage signal, and includes a two channel gating circuit which separates the signal into 2 components, one on line 140 representing the red light transmission and the other on line 142 representing the infrared light transmission. The separated signals on lines 140, 142 are filtered to remove the strobing frequency, electrical noise, and ambient noise and then digitized by an analog-to-digital converter ("ADC") section 144 (FIG. 1B). The digitized signal 146 is used by the microprocessor 148 to calculate the blood oxygen saturation.
It is well known that oxygen saturation may be computed to a useful accuracy by the formula: ##EQU1## where AC.sub.R and DC.sub.R are respectively the AC and DC components of the red transmissivity signal, AC.sub.IR and DC.sub.IR are the AC and DC components of the infrared transmissivity signal, and A, B and C are constants determined by empirical curve fitting against the results of standard blood oxygen measurements. Because the AC and DC components of the red and infrared signals correspond to the maximum and minimum amplitude values of the detected signal, the measured AC and DC signals are critical in calculating the blood oxygen saturation of hemoglobin in arterial blood. The microprocessor 148 uses the maximum and minimum voltages received from the ADC 144 to calculate the blood oxygen saturation level.
Although amplification of the detected current improves the accuracy of the oxygen saturation calculation, the added circuitry necessary for amplification increases system cost, power dissipation and the number of possible sources of errors. The embodiment shown in FIG. 1 includes amplifiers 118, 134, 126, 128 to amplify the detected signal.
An alternative method and apparatus for measuring blood oxygen saturation which does not require amplification circuitry is needed.